This proposal seeks to establish a collaboration between the Harvard School of Public Health and the Ministry of Health and the Autonomous University of Mexico on the AIDS epidemic. The focus of this program is to develop a national strategy for intervention to curtail spread of HIV infection and to characterize the virologic, epidemiologic and clinical features of the Mexican epidemic. A surveillance project will screen 30,000 adults identified in a national probabilistic survey in order to map the prevalence of infection and to project the number of future cases. The sampling frame will be used to monitor sentinel high risk populations. The medical records of all adults diagnosed with AIDS prior to 1987 will be reviewed to describe the clinical features. "The virology of HIV infection" will include viral cultures from cells of all seropositives identified in the national survey to evaluate variability in isolates and to compare to American and African strains. The antibody responses to structural and regulatory viral proteins will be assessed, as well as neutralizing activity. The presence of other retroviruses will be determined. A randomized controlled intervention trial will be conducted among 2000 homosexuals to assess the relative efficacy of intense versus usual individual and group counseling for both seropositive and negative men. A case-control study of risk factors within a high prevalence population of men and women who are paid blood donors in a slum area of Mexico City will be conducted. Since intravenous drug abuse is quite unusual in Mexico, it is likely that these reflect heterosexual transmission. The strengths of this project are considerable. These include the established collaboration, the strong infrastructure now in place in Mexico both in epidemiology and the laboratory, and the mixture of relevant expertise at the School, including virology, epidemiology, biostatistics, behavioral science and health policy. Of special merit is the presence of the Essex lab group. Of note, most of the collaborators at HSPH have been involved in the AIDS Policy Study Group of the University over the past year. Mexico stands as a prototype of the emerging Latin American countries. What is learned in this project may be generalizable to other similar populations and may be of value to our own efforts. Finally, the status of the epidemic in Mexico is several years behind that of the United States. This project should enable a more timely intervention in the spread of this deadly sexually transmitted infection.